The present invention relates to a method of simultaneously forming alkaline water of high purity and acid water of high purity using a two-chamber-type electrolytic cell. More specifically, the present invention relates to an electrolytic method of simultaneously forming (i) acid water which can be used for sterilization and disinfection and also for the treatment of skin diseases, and (ii) alkaline water having a low oxidation reduction potential which is good for drinking.
The catholyte obtained by electrolyzing municipal water with a diaphragm cell or so-called alkali ion water is said to be effective as a medicine, and is also said to have improved taste. Thus, the catholyte has enjoyed widespread use. Recently, the reduced quality of municipal water has resulted in an unpleasant odor and bad taste. As a countermeasure therefor, an apparatus for producing alkali ion water (alkaline water) and which is capable of simultaneously removing impurities and deodorizing by incorporating active carbon or a microfilter in the above-described electrolytic cell has been widely used.
On the other hand, for the production and washing of electronic parts, specially prepared sulfuric acid, hydrofluoric acid, hydrogen peroxide, hydrochloric acid, etc., has hitherto been used. However, because impurities are introduced into this system and the purification technique for removing such impurities is troublesome, a method of producing acid water for washing by a water electrolysis technique has been proposed. The electrolysis is carried out by adding chloride ion to the anode chamber of the electrolytic cell to thereby obtain an acid electrolyte having a very high oxidation reduction potential. Because the solution has a strong sterilizing action and a strong disinfecting action initially as well as after use, sodium chloride or chloride ion alone remains to the same extent as in municipal water. When the used wash solution is discarded, problems such as secondary pollution, etc., do not occur. Thus, the above described solution has been widely used for various applications.
In the water electrolysis, when ammonium chloride (NH4Cl) or sodium chloride (NaCl) for example is used as an electrolyte in the anode chamber, the anodic reaction is a disproportionation reaction represented by:
Clxe2x88x92+H2Oxe2x86x922H++ClOxe2x88x92+2exe2x88x92
The solution in the anode chamber becomes acidic with the hydrogen ion thus formed, and the resulting hypochlorous acid solution has a pH of 3 or less and an oxidation reduction potential (ORP) higher than 1.2 volts.
On the other hand, in the cathode chamber, hydroxide ion is formed and at the same time, part of the mineral components are transferred to the cathode side. The resulting drinking water which is formed as the catholyte and which can promote good health contains mineral components, has a slightly alkaline pH and has a very low ORP due to the generation of hydroxide ion and hydrogen. That is, it has been widely recognized that the decomposition of active chloride in municipal water by cathodic reduction, the reduction in oxidation reduction potential due to hydrogen generated by the electrolysis, the formation of alkali due to the hydroxy group simultaneously generated by the electrolysis, the transfer of calcium ion to the cathode side, etc., act to improve the water quality. All of this improves the taste when the catholyte is used as drinking water.
However, even if it is possible to simultaneously remove acid water from the anode chamber and alkaline water from the cathode chamber by such an electrolytic method, problems are encountered in that the separation provided by the diaphragm is not so effective, the electrolyte concentrations are increased and the acid and the alkali are mixed through the diaphragm. This results in reducing by one-half the effect of the electrolysis. Furthermore, the addition of sodium chloride to the anode chamber for obtaining stronger acid water is accompanied by the problem that the pH is slightly increased in the cathode chamber and the concentration of sodium chloride is also increased. As a result, water that is suitable for drinking is not always obtained. Also, the ORP increases in the anode chamber. However, when the electrolysis is carried out by paying attention to the increase in ORP alone, the pH of the anolyte is not sufficiently lowered and the washing effect of the water thus obtained is inadequate.
To avoid this problem, means of increasing the thickness of the diaphragm to thereby restrain the diffusion of each liquid, and also means of increasing the distance between electrodes to thereby prevent the reaction products from mixing with each other, have been proposed. However, because the electric conductivity of water is low, a large electric current cannot pass through the electrodes in such an electrolytic cell. A practical electrolytic current density is about 1 A/dm2, and even in a small-sized domestic apparatus, the electrode area must be increased by using from 3 to 5 electrodes each having an area of about 5xc3x9710 cm2. Such an electrolytic cell is disadvantageous in that the structure is complicated, the maintenance thereof takes too much time and labor, and furthermore, the electrolytic cell itself is too expensive.
To solve these problems, the present inventors previously proposed an electrolytic method capable of using a current density higher than several tens of A/dm2 by closely contacting an electrode substance with an ion-exchange membrane, and by using the ion-exchange membrane thus prepared as a solid electrolyte. The electrolytic voltage in this method was about few volts, which made electrolysis possible at a voltage far lower than that found in conventional methods. In this method, the present inventors also proposed to produce an acid water having a high ORP in the anode chamber by adding a slight amount of an acid or a salt to the anode chamber, which acid water was to be used for washing an apparatus, etc. Also, the present inventors determined that in this case, by using a non-metallic salt, a low-ORP liquid containing a non-metallic alkali such as ammonia suitable for washing semiconductors, etc., was formed in the cathode chamber.
However, although the above described catholyte and anolyte are formed, the foregoing method is inadequate for simultaneously forming liquids for washing (sterilization) and for drinking. That is, when the salt of an inorganic acid such as hydrochloric acid, sulfuric acid, etc., is added to the anode chamber, an acid and a high-ORP electrolyte suitable for washing is obtained in the anode chamber. However, the alkaline property of the catholyte becomes too high and as a result, the catholyte is unsuitable for drinking.
Also, when a metal salt is added, the above objective is almost achieved and acid water having a low pH and alkali water having a weak alkaline property is obtained. However, there is a problem in that large electric currents are required. That is, in the case of using a neutral salt such as, for example, a chloride, the ORP is regulated by the concentration of hypochlorous acid thus formed. Hypochlorous acid in a concentration of from 1 to 5 ppm is sufficient, and chlorine gas is generated if the concentration thereof exceeds ppm. If the current efficiency of chlorine generation is assumed to be 10%, a pH of about 4 to 5 is achieved by the hydrochloric acid formed in the above described reaction. To achieve a desired pH of 3 or lower, excessive electrolysis which ignores the current efficiency of chlorine generation is needed. Chlorine gas is generated when the chloride ion concentration is high, and when the chloride ion concentration is low, ozone is generated in part and the electrolysis amounts to a simple water electrolysis. On the other hand, in the cathode chamber, alkaline water having an alkalinity that is unsuitable for drinking is obtained. This is due to the metal hydroxide formed from the metal salt and the hydroxide ion that is generated in the electrolysis.
As described above, despite the fact that excessive electric power and a large electrolytic apparatus are required in conventional electrolytic methods, a means capable of simultaneously producing a catholyte for drinking water and a useful anolyte has not hitherto been achieved.
Thus, an object of the present invention is to solve the above described problems of the prior art and to provide a method of simultaneously forming, by electrolysis, an acid water having a relatively strong acidic property that is particularly suitable for washing and an alkaline water having a relatively weak alkalinity that is particularly suitable for drinking.
That is, the present invention provides a method of producing acid water and alkaline water, which comprises providing a water electrolytic cell which is partitioned by a cation-exchange membrane into an anode chamber and a cathode chamber, supplying an aqueous sodium chloride solution having a weak acidic property to the anode chamber, and conducting electrolysis to thereby obtain acid water having a high oxidation reduction potential from the anode chamber and weak-alkaline water having a pH of from 7 to 9.5 and a low oxidation reduction potential from the cathode chamber.
The present invention is described in detail below.
A characteristic feature of the present invention is the step of carrying out water electrolysis while supplying an aqueous sodium chloride solution having a weak acidic property to the anode chamber of an electrolytic cell. The aqueous sodium chloride solution having a weak acidic property is a mixture of a weak acid and an aqueous sodium chloride solution, a mixture of a diluted solution of a strong acid and an aqueous sodium chloride solution, or a solution obtained by dissolving an acid and sodium chloride in municipal water, etc. Weak acidic property means low concentration of H+ (pH=3 to 5), that is, weak acid and/or diluted strong acid. Strong acid means, e.g., HCl, H2SO4, etc., which dissociate completely in water. The electrolytic cell is a water electrolytic cell which is partitioned with a cation-exchange membrane into an anode chamber and a cathode chamber.
Typical electrolytes which are used in a conventional water electrolysis include (1) water, (2) hydrochloric acid, and (3) an aqueous sodium chloride solution. In the electrolysis of water (1), weak acid water is obtained in the anode chamber and water in a neutral range of from pH 6 to 8 is obtained in the cathode chamber depending on the operating conditions. In the electrolysis of hydrochloric acid (2), acid water is obtained in both the anode chamber and the cathode chamber. Furthermore, in the electrolysis of an aqueous sodium chloride solution (3), strong acid water is obtained in the anode chamber and strong alkaline water is obtained in the cathode chamber. As described above, conventional water electrolysis does not allow for simultaneously obtaining strong acid water in the anode chamber and weak alkaline water in the cathode chamber. Thus, one is compelled to obtain strong acid water and weak alkaline water in separate electrolytic operations which results in reduced working efficiency.
The present inventors considered that when electrolysis is carried out using a neutral electrolyte, if strong acid water is obtained in the anode chamber, then strong alkaline water is obtained from the cathode chamber. On the other hand, if weak acid water is obtained in the anode chamber, then weak alkaline water is normally obtained from the cathode chamber. Hence, when a neutral electrolyte is used, the combination of acid water and alkaline water, which is an object of the present invention, is not obtained. Thus, the present invention has been achieved by controlling the acid-alkali balance to thereby simultaneously obtain acid water and alkaline water in accordance with the above objective.
That is, by carrying out electrolysis using an aqueous sodium chloride solution having a weak acidic property as the anode feed, the acidic property obtained by hypochlorous acid formed in the anode chamber by the electrolysis of the aqueous sodium chloride solution is balanced with the alkalinity of the hydroxide formed in the cathode chamber. Furthermore, the acidic property of the anode feed supplied to the anode chamber (i.e., the aqueous sodium chloride solution having a weak acidic property) increases upon electrolysis to provide an anolyte having the desired acidic property. When the pH of the anolyte obtained by electrolysis is 4, the pH of the catholyte is about 10, although the values of pH depend upon various conditions such as the concentration of the acid that is added.
However, in the conventional electrolysis of an aqueous sodium chloride solution, strong acid water is formed in the anode chamber and strong alkaline water is formed in the cathode chamber. In order to form weak alkaline water in the cathode chamber, which is on object of the present invention, it is necessary to restrain the progress of the electrolysis. This is done by controlling the electrolysis conditions to restrain the hydroxyl ion concentration in the cathode chamber. For attaining this objective, it is most preferred to reduce the concentration of the aqueous sodium chloride solution that is supplied to the anode chamber, and the concentration of the aqueous sodium chloride solution that is supplied may be determined by calculation based on the amount and the alkalinity of the catholyte thus formed.
Also, in conventional water electrolysis, the pH is lowered and the ORP is simultaneously increased by increasing the concentration of hypochlorous acid. For example, an ORP having a sufficient sterilizing action or a sufficient disinfecting action is considered to be 1,000 mV or higher, and the concentration of hypochlorous acid needed to obtain such ORP is from about 1 to 10 ppm. On the other hand, the pH required for a washing liquid, etc., having a sterilizing action or a disinfecting action, is from about 3 to 4 and the concentration of hypochlorous acid that is needed to obtain this pH is about 1,000 ppm. That is, 99% or more of hypochlorous acid that is formed in the electrolysis is unnecessary for lowering the pH. Furthermore, if means other than above can be used for lowering the pH, the amount of electric power that is consumed becomes {fraction (1/100)} or less. In the present invention, an acid is added to a liquid that is supplied to the anode chamber from the beginning of electrolysis as described above. The pH lowering due to the foregoing acid in addition to the pH lowering by the water electrolysis is sufficient to lower the pH to the desired level without need of performing excessive electrolysis. Thus, a strong acid water satisfying the desired conditions is obtained with a minimum of electrolysis.
In other words, in conventional electrolysis a strong acid water is formed by excess electrolysis required for obtaining proper ORP. On the other hand, by improving the oxidation efficiency with chloride ion, which is most important for the disinfecting action, and by separately controlling the pH of the acid water thus obtained, the above problems of conventional electrolysis are solved in the present invention.
In the present invention, a cation-exchange membrane is used as a diaphragm to control the transfer of the ions and the electrolytes. Cations transfer from the anode chamber to the cathode chamber through a cation-exchange membrane in an applied electric field, but the transfer of hydroxide ion from the cathode chamber to the anode chamber is restrained by a cation-exchange membrane. Accordingly, in the anode chamber, the hydrogen ion concentration formed by electrolysis becomes excessive, and the anolyte becomes acidic due to the amount of other cations that are transferred. Also, in the cathode chamber, a hydroxide is formed with the cations (mainly, sodium ions from the electrolysis of an aqueous sodium chloride solution) from the anode chamber and the hydroxide ions to provide a stabilized alkaline water. In this case, the catholyte in the cathode chamber is a weak alkaline water having a pH of from about 7 to 9.5 formed by regulating the concentration of the aqueous sodium chloride solution as described above. On the other hand, the anolyte becomes a strong acid water due to the hydrogen ion generated by electrolysis of the aqueous sodium chloride solution and also due to the acid that is supplied with the aqueous sodium chloride solution from the beginning of the electrolysis.
There is no particular limitation on the cation-exchange membrane for use in the present invention if cations are selectively passed though the membrane. However, it is desirable to use a fluorine resin-type cation-exchange membrane which is stable to the acid water formed in electrolysis and which is expected to provide long life in a zero gap-type or similar electrolysis for lowering the electrolytic voltage. Also, because the electric conductivity of the electrolytes is low, the anode and the cathode are preferably disposed adjacent to the foregoing cation-exchange membrane. Furthermore, the above-described zero-gap-type electrolytic cell is constituted by closely adhering one or both of the anode and the cathode to the cation-exchange membrane such that the cation-exchange membrane is substantially a solid electrolyte. The electric conductivity of the cation-exchange membrane is generally from about 1 to 10 xcexa9/cm2, although this value may change a little depending on conditions, which is very low as compared with the electric conductivity of electrolyte (or feed solution) or water. Also, in the electrolysis at a high current density of 10 A/dm2 or higher, the electrolytic voltage is about few volts such that the electric power saving effect is large. However, there is a possibility of lowering the current efficiency of forming hypochlorous acid even though this tendency is slight. Thus, the foregoing zero-gap-type electrolysis or ordinary electrolysis may be selected depending on the intended application.
There is no particular limitation on the electrode material for use in the present invention. In particular, an anode material having a high current efficiency is desirable, namely, high chlorine generating efficiency in a diluted chloride-ion-containing solution. However, when taking into consideration current efficiency and stability, and also the stability of the anode material against dissolution, a platinum electrode is preferred. The platinum electrode is inferior to a platinum-group metal oxide electrode with respect to chlorine generating efficiency at a relatively high chloride ion concentration, but maintains good efficiency even in a low-concentration chloride solution and is very effective at a chloride ion concentration of 1,000 ppm or lower in the present invention. The platinum electrode may be a platinum-plated titanium electrode or a platinum mesh electrode, and the electrode may be selected depending on the amount of electric current that is passed through the cell.
In the present invention, by using such an electrolytic cell, the electrolysis is carried out while supplying an aqueous sodium chloride solution having a weak acidic property to the anode chamber. The aqueous sodium chloride solution having a weak acidic property is preferably prepared by adding an acid such as hydrochloric acid, etc., to an aqueous sodium chloride solution such that pH of the solution is about 3 to 4. In the case of using hydrochloric acid, the total chloride concentration is 3,000 ppm or lower and, preferably from 500 to 1,500 ppm. Namely, because the liquid supplied to the anode chamber is acidic, the current efficiency is high as compared with a neutral sodium chloride solution. However, if the chloride ion concentration is lower than 500 ppm, the current efficiency for chlorine generation tends to decrease and excessive electric current is required. Also, in the case of such an acidic aqueous sodium chloride solution, a high current efficiency for chlorine generation is obtained and hypochlorous acid is formed. However, if the foregoing chlorine concentration is over 3,000 ppm, part of the chloride forms chlorine gas which is dangerous and which can also corrode the apparatus. As a matter of course, in municipal water, etc., the extent of chlorine gas generation depends on the kind of metal ions contained therein. For example, when Fe2+ ions are present and because chlorine oxidizes Fe2+ to Fe3+, chlorine gas is not generated until all of said ions are oxidized to Fe3+ ions. Furthermore, 3,000 ppm as described above is standard in the case of using municipal water. The acid which can be added to the aqueous sodium chloride solution for use in the present invention includes hydrobromic acid and hydrofluoric acid in addition to hydrochloric acid.
In principle, water is supplied to the cathode chamber in the same amount as the liquid that is supplied to the anode chamber. By supplying water while keeping the electrolytic voltage low, alkaline water having pH of from 7 to 9.5, and preferably from 8 to 9 is obtained as a catholyte and the ORP thereof is 200 mV or lower. When the amount of water supplied to the cathode chamber is decreased, the pH of the anolyte is further increased and the ORP thereof becomes 0 or a negative voltage. However, in this case, because the resistance of the cation-exchange membrane is increased, the electrolytic voltage is slightly increased. On the other hand, when the amount of water supplied to the cathode chamber is increased, an almost contrary phenomenon occurs. That is, the pH decreases (the pH increase in reduced) and the ORP increases (the ORP decrease is also reduced). Accordingly, the pH and ORP of the alkaline water thus obtained can be controlled by controlling the amount of water that is supplied to the cathode chamber.
The solubility of calcium and magnesium contained in municipal water, etc., becomes very low under alkaline conditions, and these minerals are deposited as hydroxides. Because the concentrations of the hydroxides thus deposited are low, their influence is small. In the present invention where the acidic aqueous sodium chloride solution is prepared using municipal water, there is no problem in the anode chamber. However, in the cathode chamber where the catholyte becomes alkaline, there is a possibility that the hydroxides will deposit as described above. In particular, when the foregoing cation-exchange membrane is used as a solid electrolyte, the cation-exchange membrane may possibly become clogged with the deposited hydroxides, such that it becomes necessary to remove the deposited hydroxides.
Means for removing the deposited hydroxides include a method of directing intermittently or, preferably regularly for a short period of time the liquid supplied to the anode chamber to the cathode chamber. As a result, the liquid around the cathode and the cation-exchange membrane is made acidic, such that the deposited hydroxides are dissolved and in this case the electrolysis may proceed. However, there is a possibility that the pH of the catholyte is temporarily lowered to become an acid water. Also, the polarities of the electrodes may be regularly reversed to thereby pass a reverse current. In this case, both the supplying liquids may be reversed. However, for ease of operation, the supplying liquids are not reversed and the objective can be sufficiently obtained by reversing the electric current alone. The frequency of deposit removal depends on the electrolytic conditions and the quality of water, but usually this operation may be carried out for several minutes per 2 to 3 hours of electrolysis.
The production method of acid water and alkaline water of the present invention is described in greater detail below by reference to the following Examples, but the present invention should not be construed as being limited thereto.